Treatments using laser devices have become during the last two decades a common treatment modality in medicine. New laser technologies and delivery systems, followed by price reduction and improved quality of laser delivery systems are only a few driving forces. Some laser treatments are performed under direct irradiation in free, open space, such a laser treatment on the skin surface. However, some treatments are performed with the support of a delivery system such as transmitting the laser beam through an optical fiber or a light guide. In some of these treatments, the treatment site is characterized by a gaseous environment (e.g., during laparoscopic procedures conducted with insufflation gas).
However, some laser treatments are conducted within a liquid environment, such as kidney stone blasting or benign prosthetic hyperplasia ablation, to mention only two. From the optical perspective, the effectiveness of the delivery of energy from a laser beam to a target tissue depends, among other things, on the medium the laser passes through from its point of origin to the target tissue. In general, a liquid medium tends to absorb and scatter light more than a gaseous medium. The liquid medium may include water as a constituent, and water is known to strongly absorb light in general and infrared light wavelengths in particular.
Infrared lasers, such as Thulium, Holmium, Erbium, CO2 lasers and the like, are in common medical use in general surgery, orthopedics, and urological procedures. Since many of these procedures are conducted in the liquid environment within the body, it may be expected that a portion, perhaps even a large portion, of the laser energy emitted from an output tip of an optical fiber or a light guide may be absorbed in the liquid medium before reaching the target tissue.
However, as taught by U.S. Pat. No. 5,321,715 ('715 patent), in some circumstances, laser energy traveling in a liquid medium toward a target tissue will be absorbed, but that absorption may be less than expected. This is due to the so-called “Moses Effect”, in which the first component of the emitted energy is absorbed by the liquid and creates a bubble in the liquid medium so that the remaining energy passes through a less-restrictive or absorbing gaseous/vapor medium characterized by a lower optical attenuation.
The '715 patent describes a pulse format to increase the amount of laser energy which will arrive at the target tissue. According to the description, a first short and low energy initiation pulse is generated in order to create a bubble, followed by a higher energy treatment pulse. The second treatment pulse, when it passes through the created and now-formed bubble, experiences a lower absorption rate due to the presence of the bubble (and the absence of liquid). Moreover, the '715 patent teaches that the energy of the first bubble initiation pulse be sufficient enough to initiate the formation of a vapor bubble. The bubble thus formed may then displace a substantial portion of the fluid medium between a tip of a laser fiber and a target tissue.
The period of time between the first and second pulses can be calculated and then established based on the expected expansion rate of the bubble and the actual distance from the laser fiber tip or light guide to the target tissue. Once a bubble is generated, there are factors which control its spontaneous expansion and a second treatment pulse is then fired, according to the '715 patent, prior the bubble collapse. Van Leeuwen teaches in the prior art (“Non-contact Tissue Ablation by Holmium:YSGG Laser Pulses in Blood,” Lasers in Surgery and Medicine, Vol 11, 1991) that the bubble will expand to a diameter of about 1 mm in 100 microseconds and to 2 mm in 200 microseconds. Therefore, the '715 teaches a period shorter than 200 microseconds between the bubble initiation pulse and the following treatment pulse.
The bubble initiation pulse, based on the '715 patent, preferably is shorter than 50 microseconds and preferably shorter than 30 microseconds. In an example discussed in the '715 patent, providing a Holmium treatment laser and using a 0.5 mm fiber diameter, the bubble initiation pulse should be at least 0.02 joules—the energy required to boil water with 2.1 micron laser at the tip of the fiber. The bubble initiation pulse consumes, according to this example, 2% of 1 joule treatment pulse.
U.S. Pat. No. 5,632,739 teaches that a delay between a bubble initiation pulse and a treatment pulse is chosen so that the second pulse is emitted when the bubble size and corresponding amount of displaced fluid is at its maximum extent.
However, presently much of the pulse energy remains absorbed by the water or other biological liquid on its way to the target tissue. Non-optimal fiber end-target tissue distance may greatly affect and in fact reduce the efficiency of treatment.
The prior art, however, fails to teach a way to control and optimize the bubble expansion phase by defining, adjusting and optimizing the first initiation pulse delivered by a laser system as a function of a specific set of parameters defining a specific working envelop—total pulse energy chosen by a user for the treatment, treatment pulse repetition rate, fiber diameter and working distance from the tip of the fiber or wave guide to a target tissue and laser type. In addition, the prior art fails to teach an optimization process for determining the delay between the initiation and treatment pulses. It is one aspect of the present invention to address these shortcomings in the prior art.
Included in the solution is the optimization of treatment parameters to shape and modulate the laser pulse to provide a more effective laser-tissue interaction. This may involve optimization of pulse energy, pulse energy level(s), the number of pulses, the type and size of fiber used, and the distance of the fiber tip to the target tissue. Two pulses may be utilized so that the second pulse travels inside the bubble formed by the first of the pulses. Thus, the timing of the second pulse and any delay between the first and the second pulses may provide further optimization benefits. Further, the optimization may work in a “closed loop” mode so that the various controllable parameters can be controlled and changed on the fly to provide the most effective treatment.